Typical planar antennas are printed on high dielectric constant materials. For the case of a low temperature co-fired ceramic (LTCC), the value of the dielectric constant is fixed at 5.9, which can result in performance degradations due to substrate effects. Principally, the surface wave modes propagating in the substrate are always present, since the TM mode has a zero cut-off frequency despite the value of the substrate height, h, or its dielectric constant, .epsilon..sub.r. Higher order modes can be reduced or prevented from propagating by selecting a low value of h or .epsilon..sub.r. Recent research has developed a low loss membrane technique in semiconductor materials for use with millimeter applications. However, this is not a practical approach at microwave frequencies in the LTCC process.
The aforementioned research, reported in a 1994 article entitled W-BAND MICROSHIELD LOW-PASS FILTERS, by Stephen V. Robertson, Linda P. B. Katehi and Gabriel M. Rebeiz, 1994 IEEE MTT-S Digest, pages 625-628, describes experimental and theoretical results obtained from fabricating a planar W-band low-pass filter. The described filter is fabricated in microshield line technology, and includes a new type of planar transmission line, based on a coplanar waveguide supported by a thin dielectric membrane. The article specifically reports on the results of a planar 90 GHz low-pass filter which has been fabricated in microshield line using the aforementioned membrane technology. The device is purported to display excellent performance, including low-pass band insertion loss and low total loss. In any event, it demonstrates the efficacy of using membrane technology.